Resin-coated steel for fuel tanks of automobile and method for manufacturing the same

ABSTRACT

The present invention relates to a resin-coated steel sheet for fuel tanks of an automobile and a resin solution used for the same. The resin solution of the present invention comprises (a) a main solution of water soluble phenoxy resin having a number average molecular weight of 25,000 to 50,000; (b) 2 to 15 phr of melamine resin on the basis of the main solution; (c) 10 to 20 phr of colloidal silica on the basis of the main solution, (d) 2 to 10 phr of teflon resin on the basis of the main solution, and (e) 5 to 70 phr of metallic powder being selected at least one material from the group consisting of Al, Zn, Mn, Co, Ni, Sn and SnO. The resin solution is coated on a cold rolled steel sheet plated with zinc or zinc alloy over which a chromate layer films, and then baked at a local temperature of 160 to 250° C. to prepare a resin-coated steel sheet for fuel tanks of an automobile.

BACKGROUND OF THE INVENTION

[0001] (a) Field of the Invention

[0002] The present invention relates to an unleaded surface-treatedsteel sheet for fuel tanks of an automobile, more specifically, aresin-coated steel sheet coated with resin film on the surface ofunleaded surface-treated steel sheet coated with chromate layer and amethod of fabricating the same

[0003] (b) Description of the Related Art

[0004] Generally, it is required that fuel tanks for storing fuel of anautomobile should have resistance to corrosion on its outer surface tobe exposed to the atmosphere (hereinafter, referred to as the “cosmeticcorrosion resistance”) as well as resistance to corrosion on its innersurface to contact fuel such as gasoline (hereinafter, referred to asthe “fuel corrosion resistance”).

[0005] In addition, since two parts are joined each other in a fuel tankby seam welding or joining method such as brazing, good weldability andgood processibility are required for the steel sheet to be applied foruse in forming fuel tanks.

[0006] A ternesheet that is a cold-rolled steel sheet plated withlead-tin (Pb—Sn) alloy has widely used as such a steel sheet for fueltanks. However, the ternesheet has been limited in its use because itcontains lead (Pb) that causes environmental pollution.

[0007] In this connection, extensive researches have been made todevelop a surface-treated steel sheet for fuel tanks without any leadcontent.

[0008] Japanese patent laid-open No.63-69631 and Japanese patentpublication No.2-18982 described an electric zinc plated steel sheet asan unleaded steel sheet. The unleaded steel sheets described in theabove patent were prepared by coating zinc or zinc-based alloy such asZn—Ni, Zn—Co, Zn—Fe, Zn—Al on the surface of steel sheet in the amountof 1-200 g/m² and coating organic resin film comprising phenoxy resinand rubber-modified epoxy on the upper side of zinc plated layer in thethickness of 2-50 μm.

[0009] The above surface-treated steel sheet, however, has a problemsuch as exfoliation of plated layer while processing because the platedlayer is thick. In addition, since an organic coating layer coated onthe uppermost layer is also thick, it is difficult to weld. Furthermore,adhesion between zinc alloy plated layer and organic resin coated layerbecomes lower so that there is a problem of exfoliation between the twolayers.

[0010] In order to solve the above problems, the present inventorsinvented an International patent publication No. WO 00/32843. The abovepatent provides an unleaded plated steel sheet coated with resin coatingcomprising metal powder on chromate layer that is in turn coated on zincor zinc-based alloy plated steel sheet.

[0011] The above invention, however, has a problem that since processedside does not resist against the pressure of mold die while processingdue to metal powder comprised in resin coating, it may exfoliate a partof resin layer

[0012] In the coated resin coating, the above problem results from lowerslip characteristic of resin layer. The slip characteristic of resinlayer has a close relation with wax added to resin solution, and a type,composition and particle size of metallic powder.

[0013] Therefore, an unleaded steel sheet that can prevent exfoliationof coating by improving slip characteristic as well as maintainingcosmetic corrosion resistance and fuel corrosion resistance is required.

SUMMARY OF THE INVENTION

[0014] Therefore, the present invention is derived to solve the aboveproblems. The present invention provides an improved resin solution usedfor preparing resin-coated steel sheet without damaging chemicalcharacteristics of resin.

[0015] It is another object of the present invention to provide a methodof fabricating resin-coated steel sheet for fuel tanks of an automobilewhich has improved cosmetic corrosion resistance and fuel corrosionresistance as well as processibility by coating the resin solution.

[0016] In order to achieve the above objects, the resin solution of thepresent invention is prepared by mixing a main solution selected fromepoxy resin, urethan resin, and phenoxy resin, melamine resin, colloidalsilica, teflon-based wax, and at least one metallic powder selected fromAl, Zn, Mn, Co, Ni, Sn and SnO.

[0017] The main solution used in the resin solution of the presentinvention is a water-soluble phenoxy resin with a number averagemolecular weight of 25,000-50,000. Melamine resin is added as a hardenerand the amount is 2-15 phr on the basis of the main solution. Also, anamount of colloidal silica is 10-20 phr on the basis of the mainsolution, an amount of teflon wax is 2-10 phr on the basis of the mainsolution, and an amount of metallic powder is 5-70 phr on the basis ofthe main solution.

[0018] The teflon-based wax added to the resin solution of the presentinvention is preferably a powder type and particle size of 0.1-3 μm. Inaddition, particle size of metallic powder is 0.5-5 μm.

[0019] The present invention provides a method of fabricatingsurface-treated steel sheet by using the resin solution.

[0020] The surface-treated steel sheet of the present inventioncomprises coating chromate film on a cold-rolled steel sheet plated withzinc or zinc alloy and then it is coated with resin solution of thepresent invention. After coating the resin solution, it is baking driedat 140-250° C. based on metal temperature. In this case, the coatingthickness of resin solution is preferably 1-10 μm based on coatingthickness after drying.

[0021] The surface-treated steel sheet of the present invention canmaintain slip characteristic of the resin coating as well as preventexfoliation of coating resulted from metallic powder.

[0022] Therefore, if the surface-treated steel sheet of the presentinvention is used to manufacture a fuel tank, a press processibility ofthe steel sheet is remarkably improved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a cross-sectional view of a resin-coated steel sheet forfuel tank wherein one side is coated with the resin.

[0024]FIG. 2 is a cross-sectional view of a resin-coated steel sheet forfuel tank wherein both sides are coated with the resin.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Preferred embodiments of this invention will be explained withreference to the accompanying drawing.

[0026] As shown in FIG. 1, the resin-coated steel sheet according to thepresent invention has a structure that zinc or zinc-nickel alloy isplated on a cold-rolled steel sheet and chromate is coated on the platedsteel sheet, and resin solution of the present invention is coated inthe thickness of 1-10 μm.

[0027] Low-carbon steel sheet having less than or 0.03% of carboncontent is preferably used as the cold-rolled steel sheet in the presentinvention.

[0028] Zinc (Zn), zinc—nickel (Zn—Ni) alloy, zinc—cobalt (Zn—Co) alloy,zinc—manganese (Zn—Mn) alloy or zinc—chrome (Zn—Cr) alloy may be used asthe plating material. In the present invention, zinc—nickel (Zn—Ni)alloy steel sheet is preferably used because it has better cosmeticcorrosion resistance than that of zinc-plated steel sheet.

[0029] Chromate solution applied to zinc-nickel plated steel sheetincludes a reactive type, electrolyte type, and coating type, in whichcoating type is preferable in terms of cosmetic corrosion resistance.When the chromate solution is applied to the steel sheet, it can beplated on one side or both sides. It is preferable to coat on bothsides.

[0030] However, in case of resin solution, it can be alternativelycoated on one side(FIG. 1) or both sides(FIG. 2) depending on the needsof a secondary processor.

[0031] Such an alternation depends on the welding condition when theresin-coated steel sheet is used for manufacturing fuel tanks. That is,in case of high current condition that is easy for welding and frequentreplacing of electrolyte, it is preferable to use steel sheet coated onboth sides whereas in case of low current condition and infrequentreplacing of electrolyte, it is preferable to use steel sheet coated onone side.

[0032] When manufacturing a fuel tank by a steel sheet coated with resinon one side, it is preferable to weld the tank by placing the coatedside to face fuel side and chromate-coated side which is not coated withresin to face outer side. Thus, it makes welding easily because thewelding electrolyte does not contact with resin part. In addition, ifnecessary, paint is applied to a side which is not coated with resin inthe thickness of about 100 micrometer so to reinforce corrosionresistance of fuel lank, thus, it has little influence to corrosionresistance.

[0033] Hereinafter, the resin solution used in the resin-coated steelsheet of the present invention will be explained in detail.

[0034] The resin solution of the present invention comprises a mainresin solution, a hardener, colloidal silica, metallic powder, and alubricant.

[0035] The main solution of the resin solution is acryl resin, epoxyresin, or urethan resin, preferably phenoxy resin.

[0036] Phenoxy resin has excellent cosmetic corrosion resistance andfuel corrosion resistance because it has higher glass transitiontemperature(100° C., Tg) than other resins.

[0037] Thus, even if the surrounding temperature of a fuel tank is morethan 100° C., the chains of phenoxy resin do not show microbrownmovement and do not modify. Due to these characteristics of phenoxyresin, it prevents water or gasoline from penetrating so that cosmeticcorrosion resistance of steel sheet is reinforced.

[0038] It is preferable to use phenoxy resin having a number averagemolecular weight of 25,000 to 50,000. When the number average molecularweight is below than 25,000, it is difficult to have desired cosmeticcorrosion resistance. When the number average molecular weight is morethan 50,000, it is impossible to synthesize the resin.

[0039] When epoxy resin is a main solution, however, it is preferable tohave a number average molecular weight of 4,000 to 6,000 and whenurethan resin is a main solution, it is preferable to have a numberaverage molecular weight of about 15,000.

[0040] Melamine resin as a hardener can be added to phenoxy resinsolution of the present invention. Melamine resin has a role to reactwith hydroxy group of phenoxy resin while forming coating layer so thatit makes a closer resin coating. That is, linear structure of phenoxyresin converts to network structure by adding melamine resin. Thus, thisresin structure can prevent outer corrosion element from penetrating sothat it can improve cosmetic corrosion resistance.

[0041] The content of melamine resin is preferably 2-15 phr(parts perhundred resin: an amount per 100 weight part of main solution) based onphenoxy resin. If content of melamine is less than 2 phr, hardeningreaction is not sufficient. On the contrary, if content of melamine ismore than 15 phr, reactions between hardeners themselves occur so thatit forms cracks in the coating layer.

[0042] Colloidal silica is added to improve cosmetic corrosionresistance of resin coating. Since soluble phenoxy resin is basic,colloidal silica which is also basic is selected from other silica.

[0043] The content of colloidal silica is preferably 10-20 phr based onphenoxy resin content. If content of colloidal silica is less than 10phr, it is too small to have cosmetic corrosion resistance effect. Onthe contrary, if content of colloidal silica is more than 20 phr, thereis no improved effect of cosmetic corrosion resistance relative to theadded content of colloidal silica.

[0044] Metallic powder added into the resin solution of the presentinvention has a role to enhance electrically conductive property ofresin coating.

[0045] Since resin coating itself serves as a nonconductor whileresistance welding between sheet and sheet, sparks can occur duringwelding or resin coating of welded part can be exfoliated easily.Therefore, it needs to penetrate metallic powder into the inner side ofresin coating in order that it can use both screening effect of resinand electric conductivity of metallic powder. Thus, the resin coatingcan maintain both screening effect and electric conductivity.Accordingly, the resin solution satisfies weldability and cosmeticcorrosion resistance of the steel sheet simultaneously. Metallic powderis preferably selected from metals having conductivity as well ascosmetic corrosion resistance and fuel corrosion resistance.

[0046] Examples of metallic powder are Al, Zn, Mn, Co, Ni, Sn, and SnO.These metals can be used alone or combination of at least one.

[0047] A particle size and a type of the metallic powder added intoresin solution have important effect.

[0048] A particle size of the metallic powder is preferably 0.5-5 μm. Ifthe particle size is less than 0.5 μm, dispersion decreases in the resinsolution, secondary agglomeration occurs and cost increases. On thecontrary, if the particle size is more than 5 μm, the particle is tooheavy so that it sinks in the resin solution, which results sludge. Inaddition, the sludge protrudes into the surface of resin coating so thatit deteriorates the processibility of steel sheet.

[0049] A particle type of metallic powder is preferable to have aplate-type rather than a sphere-type in terms of conductivity of resincoating and stability of solution because sphere-type more easilyprecipitates in the resin solution than plate-type. In addition, interms of conductivity, since a plate-type has more chances to overlap,it has a role as a path of electrical conductivity. The thickness ofplate-type particle is preferably 0.1-0.5 μm.

[0050] The content of metallic powder is preferably 5-70 phr based onphenoxy resin content. If content of metallic powder is less than 5 phr,it cannot contribute to weldability. On the contrary, if content ofmeetallic powder is more than 70 phr, storability of resin coatingsolution decreases and coating adhesion with chromate layer decreases.

[0051] Wax added into resin solution serves as a lubricant to metallicpowder. The wax is preferable to usepolytetrafluoroethylene(hereinafter, “tefron”).

[0052] Comparing the prior ethylene-based wax, tefron-based wax has anexcellent slip characteristic of resin coating. In addition,tefron-based wax can cover the metallic powder protruded onto resincoating so that it can prevent friction between die and resin coatingwhile press processing.

[0053] The content of tefron wax is preferably 2-10 phr based on phenoxyresin content. If content of tefron wax is less than 2 phr, it is toosmall to improve surface friction coefficient. On the contrary, ifcontent of tefron wax is more than 10 phr, coating adhesion with paintwhich can be applied into upper resin layer decreases.

[0054] A particle size of tefron wax added into resin solution hasimportant effect. A particle size of tefron wax is preferably 0.1-3 μm.If the wax size is less than 0.1 μm, the ball-bearing effect based onwax theory decreases in the resin solution. On the contrary, if the waxsize is more than 3 μm, the stability of resin solution decreases and itprevents metallic powder from forming current structure so thatconductivity decreases.

[0055] Hereinafter, a method of fabricating resin-coated steel sheetusing resin is solution of the present invention is explained.

[0056] At first, zinc-nickel plating that is plated on the surface ofcold-rolled steel sheet is explained.

[0057] Although there are many kinds of plating methods as zinc-nickelplating method, electric plating method is used in the present inventionbecause it is easy to control an amount of plating and it has goodsurface property after plating.

[0058] When plating with zinc-nickel alloy, the content of nickel is10-14 wt % because alloy having the above composition is excellent interms of processibility and cosmetic corrosion resistance.

[0059] The amount of plating of zinc-nickel alloy that is plated oncold-rolled steel sheet is preferably 10-40 g/m². If the amount ofplating is less than 10 g/m², the cosmetic corrosion resistance is notsufficient. On the contrary, if the amount of plating is more than 40g/m², the thickness of plating is too thick so that alloy-plated layeris exfoliated and powdering occurs while press processing. In addition,as the plating amount increases, power used for welding increases.

[0060] After plating zinc-nickel alloy, chromate layer is coated.

[0061] Chromate layer is coated in order to increase adhesion betweenresin-coated layer and zinc-nickel plated layer.

[0062] Chromate solution used for chromate coating comprises a) a mainsolution prepared by adding 20 to 150 w % of phosphoric acid, 10 to 100w % of fluoric acid, 50 to 2000 w % of colloidal silica of which ph is2-5, and 5 to 30 w% of sulfuric acid on the basis of chrome in a chromesolution having 0.4 to 0.8 of trivalent chrome rate; b) a hardeneraqueous solution comprising epoxy silane of 2 to 10 w % on the basis oftotal hardener solution, the pH of which being controlled between 2 and3, said hardener aqueous solution of 5 to 50 w % being added to the mainsolution.

[0063] Coating process of chromate layer includes roll coating, spray,impregnation and so on. It is preferably to use roll coating process inthe present invention.

[0064] The coating process comprises dipping chromate solution in a drippan into pick-up-roll(P.U.R), transferring it by a transfer-roll(T.F.R),dipping it into steel sheet in the applicator-roll(A.p.R), and drying.The amount of chromate solution attached to the steel sheet is regulatedby each roll driving direction, rolling speed, and each roll adhesionpressure. The above roll coating process can be applicable to one sideor both sides of steel sheet.

[0065] A steel sheet coated with chromate solution is baking dried at adrying furnace. Baking temperature of the steel sheet coated withchromate is preferably 140-250° C. based on the metal temperature. Ifbaking temperature is less than 140° C., hardening reaction of chromatesolution is not sufficient. On the contrary, if the baking temperatureis more than 250° C., minute cracks occur on the chromate coating layerso that cosmetic corrosion resistance decreases.

[0066] The amount of chromate is preferably 20-150 mg/m² on the basis ofchrome amount after drying. If the content is less than 20 mg/m²,cosmetic corrosion resistance is not sufficient so that it is notappropriate to use as a fuel tank. On the contrary, If the content ismore than 150 mg/m², chrome elutes from chromate coating layer and thecost increases so that it is not economical.

[0067] As above, after coating chromate layer, the resin solution of thepresent invention is coated on the steel sheet.

[0068] The composition of resin solution is the same as the above andcoating process is the same as that of the chromate solution.

[0069] After coating, baking temperature of the steel sheet coated withresin solution is preferably 140-250° C. based on the metal temperature.If baking temperature is less than 140° C., hardening reaction of resinsolution is not sufficient so that cosmetic corrosion resistance andfuel corrosion resistance decrease. On the contrary, if the bakingtemperature is more than 250° C., hardening reaction does not occur anylonger and loss of calories increases.

[0070] Thickness of the resin coating which is coated on upper side ofchromate layer is preferably 1.0-10.0 μm. If the thickness is less than1.0 μm, the thickness of the coating is too thin to have sufficientcosmetic corrosion resistance and fuel corrosion resistance. On thecontrary, if the thickness is more than 10.0 μm, there is no influenceto cosmetic corrosion resistance and fuel corrosion resistance accordingto increase of the thickness as well as weldability lowers when steelsheet is welded each other.

[0071] Now, preferred embodiments are suggested to help the apparentunderstanding of the present invention. The below embodiments areprovided for the sake of clear understanding only and the presentinvention is not limited thereto.

EXAMPLE

[0072] After plating zinc-nickel alloy on cold-rolled steel sheet in theamount of 30 g/m², chromate was coated on there. Chromate coating layerwas coated in the amount of 50 mg/m² based on the chrome amount afterdrying and baking dried at 160° C.

[0073] In this case, chromate solution has 0.5 of trivalent chromeratio(chrome reduction ratio) and 29 g/l of chrome concentration. Itcomprises a main solution comprising 100 wt % of colloidal silica, 30 wt% of fluoric acid, 50 wt % of phosphoric acid, and 10 wt % of sulfuricacid based on chrome and 30 wt % of hardener solution comprising 10 wt %of epoxy silane.

[0074] Resin solution dispersed in water was coated on the steel sheetas a phenoxy resin. The composition of resin solution includes 100 g ofphenoxy resin with number average molecular weight of 50,000(Unioncarbide, PKHW-35), 5 phr of melamine resin as a hardener(Cytec company,Cymel 325), 15 phr of colloidal silica with particle size of 20 nm(IlsanChemical Company, Snowtex-N) and 15 phr of metallic powder with particlesize of 2 μm. In addition, according to the composition of below table1, wax was added into the above resin solution. The wax was atefron-based wax made in Korea, Okitusmo company(NLF25W). To compare thematerial properties of the resin coated steel sheet of the presentinvention, ethylene-based wax(Korea, PS35) was used instead oftefron-based wax.

[0075] The above resin solution was coated on steel sheet and bakingdried at 190° C. As a result, resin-coated steel sheet with driedcoating thickness of 3 μm was manufactured.

[0076] A friction coefficient of steel sheet, an exfoliation of resincoating layer, stability of resin solution, and coating adhesion of topcoater were measured and the results were shown in the table 1.

[0077] The friction coefficient, the exfoliation of resin coating layer,stability of resin solution, and coating adhesion of top coater wereevaluated by following method.

[0078] 1) Friction Coefficient

[0079] The prepared resin-coated steel sheet was cut into 45×300 mm andthe burrs of the corners were removed. The friction coefficient wasmeasured under 0.27 kg/cm² of pressure and 1.000 mm/min of drawing speedin the one-side friction tester. The results are as follows.

[0080] ⊚: friction coefficient; less than 0.15

[0081] : friction coefficient; 0.15-0.2

[0082] □: friction coefficient ; 0.2-0.25

[0083] Δ: friction coefficient ; 0.25-0.3

[0084] X: friction coefficient; more than 0.3

[0085] 2) Exfoliation of Coating layer

[0086] After measuring friction coefficient, the extent of scratch andexfoliation were measured by scratching the specimens. The results areas follows.

[0087] ⊚: The exfoliated and scratched area is 0%.

[0088] : The exfoliated and scratched area is 0-5%.

[0089] □: The exfoliated and scratched area is 5-10%.

[0090] Δ: The exfoliated and scratched area is 10-20%.

[0091] X: The exfoliated and scratched area is more than 20%.

[0092] 3) Stability of Resin Solution

[0093] Stability of resin solution was measured by time taken to formsludge in the resin solution without any oscillation at normaltemperature. The results are as follows.

[0094] ⊚: The time taken to form sludge is more than 5 days.

[0095] : The time taken to form sludge is 1-5 days.

[0096] □: The time taken to form sludge is 12-24 hours.

[0097] Δ: The time taken to form sludge is 2-12 hours.

[0098] X: The time taken to form sludge is less than 2 hours.

[0099] 4) Coating Adhesion of Top Coater

[0100] Coating adhesion between resin coating layer and top coater wasmeasured by coating melamine-alkyd resin on the top of resin coatingwith a bar coater and drying for 5 min at normal temperature. And then,the steel sheet coated with melamine-alkyd resin was baking dried at150° C. for 20 min. The thickness of dried coating was 20 μm.

[0101] After coating the top coater, cross stripes were made by thespecimens with 1 mm spaces. After attaching cellophane tape into thecoating and adding determined pressure, the tape was peeled off. Thecoating area exfoliated was measured. The results are as follows.

[0102] ⊚: The area of top coater exfoliated is 0%.

[0103] : The area of top coater exfoliated is 0-5%.

[0104] □: The area of top coater exfoliated is 5-10%.

[0105] Δ: The area of top coater exfoliated is 10-20%.

[0106] X: The area of top coater exfoliated is more than 20%. TABLE 1wax Quality measurement Particle Amount Friction Coating Solution No.Type size(μm) (phr) coefficient exfoliation stability Adhesion Com. 1 —— 0 X Δ ⊚ ⊚ Com. 2 Tefron 0.1 1 □ □ ⊚ ⊚ Ex. A 2 ⊚ ⊚ ⊚ ⊚ Ex. B 5 ⊚ ⊚ ⊚ ⊚Ex. C 10 ⊚ ⊚ ⊚ ⊚ Com. 3 15 ⊚ ⊚ ⊚ X Com. 4 0.3 1 □ □ ⊚ ⊚ Ex. D 2 ⊚ ⊚ ⊚ ⊚Ex. E 5 ⊚ ⊚ ⊚ ⊚ Ex. F 10 ⊚ ⊚ ⊚ ⊚ Com. 5 15 ⊚ ⊚ ⊚ Δ Com. 6 1.5 1 □ 0 ⊚ ⊚Ex. G 2 ⊚ ⊚ ⊚ ⊚ Ex. H 10 ⊚ ⊚ ⊚ ⊚ Com. 7 15 ⊚ ⊚ ⊚ X Com. 8 3 1 □ 0 ⊚ ⊚Ex. I 2 ⊚ ⊚ ⊚ ⊚ Ex. J 10 ⊚ ⊚ ⊚ ⊚ Com. 9 15 ⊚ ⊚ ⊚ Δ Com. 10 5 1 □ 0 0 ⊚Com. 11 2 ⊚ ⊚ □ ⊚ Com. 12 10 ⊚ ⊚ Δ ⊚ Com. 13 15 ⊚ ⊚ X X Com. 14 Ethylene0.1 1 □ Δ ⊚ ⊚ Com. 15 2 0 □ ⊚ ⊚ Com. 16 5 ⊚ □ ⊚ ⊚ Com. 17 10 ⊚ 0 ⊚ ⊚Com. 18 15 ⊚ 0 ⊚ Δ Com. 19 0.3 1 0 Δ ⊚ ⊚ Com. 20 2 0 □ ⊚ ⊚ Com. 21 5 ⊚ □⊚ ⊚ Com. 22 10 ⊚ 0 ⊚ ⊚ Com. 23 15 ⊚ 0 ⊚ Δ

[0107] As can be seen in table 1, the quality of surface-treated steelsheet of tefron-based wax is more excellent than that of ethylene-basedwax. Particularly, as can be seen from the coating exfoliation results,it is preferable to use a tefron-wax with particle size of 0.1-3.0 μmand the amount of 2-15 phr in order to manufacture steel sheet for fueltank with improved press processibility.

What is claimed is:
 1. A resin solution used for preparing resin-coatedsteel sheet for a fuel tank of an automobile comprising: a main resinsolution selected from epoxy resin, urethan resin and phenoxy resin;melamine resin; colloidal silica; tefron-based wax; and at least oneplate-type metallic powder selected from Al, Zn, Mn, Co, Ni, Sn and SnO.2. The resin solution of claim 1, wherein said main resin solution iswater-soluble phenoxy resin having a number average molecular weight of25,000 to 50,000; said melamine resin is added in the amount of 2 to 15phr on the basis of said main solution; said colloidal silica is addedin the amount of 10 to 20 phr on the basis of said main solution; saidtefron-based wax is added in the amount of 2 to 10 phr on the basis ofsaid main solution; and said metallic powder is added in the amount of 5to 70 phr on the basis of said main solution.
 3. The resin solution ofclaim 2, wherein said tefron-based wax has a particle size of 0.1-3 μm.4. The resin solution of claim 3, wherein said metallic powder has aparticle size of 0.5-5 μm.
 5. A method of fabricating resin-coated steelsheet for a fuel tank of an automobile comprising the steps of: coatinga resin solution comprising a main resin solution of phenoxy resinhaving a number average molecular weight of 25,000 to 50,000; 2 to 15phr of melamine resin on the basis of said main solution; 10 to 20 phrof colloidal silica on the basis of said main solution; 2 to 10 phr oftefron-based wax on the basis of said main solution; and 5 to 70 phr ofat least one plate-type metallic powder selected from Al, Zn, Mn, Co,Ni, Sn and SnO; and baking drying said resin-coated steel sheet at140-250° C.
 6. The method of fabricating resin-coated steel sheet ofclaim 5, wherein thickness of said resin coating is 1-10 μm based ondried coating thickness.
 7. The method of fabricating resin-coated steelsheet of claim 6, wherein the particle size of tefron-based wax of saidresin solution is 0.1-3 μm.
 8. The method of fabricating resin-coatedsteel sheet of claim 7, wherein the particle size of metallic powder ofsaid resin solution is 0.5-5 μm.
 9. A resin-coated steel sheet for afuel tank of an automobile comprising a main resin solution ofwater-soluble phenoxy resin having a number average molecular weight of25,000 to 50,000; 2 to 15 phr of melamine resin on the basis of saidmain solution; 10 to 20 phr of colloidal silica on the basis of saidmain solution; 2 to 10 phr of tefron-based wax on the basis of said mainsolution; and 5 to 70 phr of at least one of metallic powder selectedfrom Al, Zn, Mn, Co, Ni, Sn and SnO on the basis of said main solutionand with 0.5-5 μm of particle size, said resin solution coated in thethickness of 1-10 μm based on dried coating thickness.